It is generally recognized by those skilled in the art of electrical dimming control systems for fluorescent lamps that the externally heated cathodes of rapid start type fluorescent lamps must be heated to a temperature that permits the required level of thermionic electron emission to be achieved. For this reason, such fluorescent lamp dimming control systems usually provide for initially turning the lamps "full-on" so that the rated arc current flows, before dimming, i.e., reduction of the arc current, is undertaken. Such full-on ignition of the lamps is generally accomplished by applying full rated line voltage to the standard transformer-ballast usually employed as the lamp driver. This approach is described, for example, in U.S. Pat. No. 4,350,935 (see column 12 lines 27-35) and U.S. Pat. No. 4,352,045 (see column 7 starting at line 5). When full rated A.C. line voltage is applied to the ballast driving the fluorescent lamp load, the cathode heating voltage as well as the necessary arc striking voltage appear at the lamp electrodes at the specified nominal magnitudes. After a short heating period, the cathode begins to emit electrons, and the arc thereafter ignites and extinguishes one or more times before the cathode reaches the temperature at which the thermionic emission provided is capable of sustaining the arc at the rated current. This initial arc-on/arc-off operation causes the cathode to "sputter" which substantially contributes to cathode wear. The term "sputtering" as used here refers to the actual physical emission or giving off of cathode material from the remainder of the cathode caused when arc current flows to the cathode prior to the temperature of the cathode reaching a value which insures sufficient electron emission. Thus the cathode is, in effect, operating in a temperature-limited mode rather than in a space-charge-limited mode as intended.
Cathode wear is the primary determinant of the life of a fluorescent lamp because when the cathode is finally consumed, insufficient emission electrons are available to ignite or maintain the arc. Nevertheless, this ignition wear phenomena is accepted in the prior art. The lamp manufacturing industry generally rates a standard 40 watt lamp as having a 20,000 hour Mean Time Between Failure (MTBF) life based on a test cycle of three hours "on" and twenty minutes "off". If it also well known that lamp operating life will be extended when longer "on" periods are provided between the starting events which cause the cathode wear.